MPEX promises a versatile tool for fusion reactor materials development

Temperatures hotter than the center of the sun. Magnetic fields hundreds of thousands of times stronger than the earth’s. Neutrons energetic enough to change the structure of a material entirely.

These are not the conditions on a faraway planet or deep inside the earth’s core—they’re the normal demands inside a fusion reactor. This extreme environment puts scientists and engineers up against a challenge nearly as great as the puzzle of producing fusion itself: creating materials tough enough to withstand fusion conditions over long periods of time.

“If you want to have fusion power, and eventually a fusion reactor that runs most of the time, that will have a big impact on your material requirements,” said Juergen Rapp, the chief scientist for the Material Plasma Exposure eXperiment (MPEX) project at Oak Ridge National Laboratory. “You will chew through material with steady-state operation over many years. For a long time, physicists studied the effect of the materials on the plasma, but now in the era of ITER and looking forward, we have to study the effect of the harsh conditions of a burning plasma on the materials.”

With that goal in mind, researchers in the Nuclear Science and Engineering Directorate at ORNL are preparing a design for MPEX, a next-generation linear plasma device that will support study of the way plasma will interact long term with the components of future fusion reactors—in particular, the divertor, the power and particle “exhaust system” of a fusion reactor. MPEX recently completed a Department of Energy Critical Decision-1 assessment, moving it one step closer to the start of construction.

In a recent community report to the Office of Science Fusion Energy Sciences Advisory Committee, the US fusion community identified materials research as one of the highest priorities on the path to commercial-scale fusion energy, emphasizing that materials and fusion technology will drive the arrival timeline of a prototype power plant.

ORNL conducts a wide range of materials science research for DOE’s Office of Science and is also the location of US ITER, which has been a driver for fusion technology. The international ITER project, now under construction in France, will demonstrate a 500 MW burning plasma.

“We have nearly a 70-year history in fusion energy research and development at ORNL,” said Alan Icenhour, associate laboratory director for nuclear science and engineering at ORNL. “Because of that extensive foundation and the current collection of diverse fusion expertise, this is the ideal location for discovering how to advance fusion materials beyond what we have available today.”

Achieving commercial-scale fusion energy is considered one of the greatest engineering challenges of the 21st century. Fusion has the potential to provide a limitless source of carbon-free energy. As fusion confinement devices and technology transition from pulsed to steady-state operation—essential for a viable power plant—new materials challenges arise. MPEX will be a vital research tool to address those challenges.

“Our work on plasma-material interaction with MPEX is the bridge between burning plasma physics and the materials science knowledge we have obtained from small material samples,” Rapp said.

MPEX plasmas will be produced by a high-power radio-wave source, known as a helicon source. It will efficiently create large-scale volumetric plasmas with high density, putting it ahead of rival source concepts. The plasma is then heated with state-of-the-art microwave systems and radio-frequency waves to reach fusion reactor conditions. So far, no other linear plasma device has this capability. A removable target chamber at the far end of the linear device will house the material sample. The removable chamber will allow exposed samples to be studied in greater detail at multiple facilities and will be adjustable so that the plasma can interact with samples at a low angle of incidence—a key feature for divertor research.

ORNL’s extensive nuclear capabilities, including operation of the High Flux Isotope Reactor, drive groundbreaking projects like MPEX. Scientists plan to use HFIR to expose materials to neutron fluxes comparable with that of a fusion reactor and then place those samples into MPEX. Doing so will provide a complete picture of the combined effect that irradiation, high heat loads and large magnetic fields have on potential reactor component materials.

“At ORNL, we’re familiar with irradiated materials, thanks to our nuclear history. We are designing for that from the beginning,” said Phil Ferguson, the MPEX project director. “We’ll design MPEX to handle 10–50 dpa (displacement per atom) samples. Then we’ll start to get a true feeling for the interaction of plasma with neutron irradiation and the impact on materials.”

The breadth of materials research performed at ORNL opens the door to study the effects of fusion conditions on novel materials developed in-house. Materials of interest include variations of carbon fiber and ceramics.

To create the extreme fusion conditions necessary for materials research, a waveguide for electron cyclotron heating waves—which are produced by a high power microwave-generating device called a gyrotron—will run perpendicular to the MPEX chamber. In the chamber, a helical antenna, delivering ion cyclotron heating waves, will be attached. These systems will work in tandem to heat plasmas up to 150,000 K. A host of low-temperature superconducting magnets will create a magnetic field of 2.5 T inside the chamber, and a large vacuum system will ensure a clean plasma, free of contaminants. Each of these systems will work together to expose materials to a close replica of a steady-state fusion reactor.

“MPEX is expected to exceed the conditions of any current device,” Ferguson said. “It can go all the way to fluence levels in DEMO—the machine after ITER—which are relevant for a prototype fusion reactor.”

The path to viable fusion energy is complex and requires researchers to take advantage of current machines and experiments, supercomputing and whole-device modeling—all of which is done at ORNL—while continuing to advance foundations in plasma physics.

“We use high-performance computing to better extrapolate to future reactor conditions,” Ferguson said. “The facilities that exceed ITER and lead us to fusion energy will be designed computationally.”

At the same time researchers are studying plasma characteristics and pushing the performance of fusion technology on current fusion machines. New reactors, such as ITER, are under construction, and future reactors, such as a steady-state prototype machine, have not yet been designed. Put simply, the materials challenge has been sufficiently solved for current devices, but efforts are just now ramping up for future projects like a prototype reactor.

“For MPEX, we don’t want a one-trick pony,” Rapp said. “We want a versatile device that can cover a large part of the materials problem.”

MPEX represents a shift from the historical direction of the plasma-material interaction field, which for many years focused on the effect that materials had on plasma, but not on the effect that plasma had on materials. Scientists were intent on creating the purest possible plasma in order to foster the most efficient fusion reaction.

That goal is no less of a priority today, but new research about plasma-reformed surfaces, or altered atomic structures after exposure to plasma, has shown that degradation of materials inside a fusion reactor can have significant consequences.

“From a structural engineering point of view, the sample after plasma reforming is not what I put into the device,” Ferguson said. “So how can I have confidence for material to work in a fusion reactor long enough for efficient power production? That’s what the linear device does for us.”

The MPEX device represents the latest piece in the broader fusion puzzle ORNL has been working diligently to solve for years. The story of fusion energy at ORNL spans multiple continents, and numerous fusion devices, including tokamaks and stellarators. ORNL’s nuclear expertise touches nearly every component of a fusion reactor, from the fundamental physics of burning plasmas to the cutting-edge technologies required to make industrial-scale fusion a reality.

Each piece of that puzzle is a significant undertaking on its own, pushing the limits of technology and demanding new innovations. Fuel cycle and tritium-breeding research are required to ensure that a fusion power plant is self-sustaining. A vast array of powerful heating systems, such as the electron and ion cyclotron heating that will be featured on MPEX, must be constantly improved to reach the astronomical temperatures required to sustain fusion. Diagnostic tools must be designed and tested to not only have the capability to control a complicated fusion device but also to survive the harsh environment that they create. All of these are active research areas that form the backbone of the fusion program at ORNL.

Fusion experts at ORNL also make substantial contributions to current experimental collaborations around the world. In addition to the ITER project, the laboratory continues to leave its footprint at facilities like DIII-D in San Diego, Joint European Torus (JET) in the UK, Wendelstein-7X in Germany, KSTAR in South Korea and many others—yielding exchanges of knowledge that benefit both the collaboration and ORNL.

As the fusion community anticipates burning plasma science on ITER and contemplates commercially viable fusion power, MPEX will answer some crucial questions. For the researchers at ORNL who have had their sights set on fusion power for the better part of a century, that is an accomplishment worth celebrating.

MPEX is supported by DOE’s Office of Science.

UT-Battelle manages ORNL for the Department of Energy’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit

—Adriana Ghiozzi, Lynne Degitz

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